UNITED NATIONS, July 18 (Reuters) - The United Nations condemned rocket fire into Israel from Gaza that ended a five hour humanitarian truce, but is "alarmed by Israel's heavy response," U.N. political affairs chief Jeffrey Feltman told the U.N. Security Council on Friday.

"Israel has legitimate security concerns, and we condemn the indiscriminate rocket fire from Gaza into Israel that ended yesterday's temporary ceasefire. But we are alarmed by Israel's heavy response," Feltman told the emergency council session.

Israel intensified its land offensive in Gaza with artillery, tanks and gunboats on Friday and warned it could "significantly widen" an operation Palestinian officials said was killing ever greater numbers of civilians.